FIG. 1 shows two telephone sets 301 and 302 for voice communications which are connected a communication path 303, such as a telephone network or a voice over Internet protocol (VoIP) network. On the communication path 303, four devices 310A, 310B, 310C, and 310D are provided with an example. These four devices 310A to 310D might cause an echo when they fall in failure or disorder.
Since an echo disturbs clear voice conversations on a cell phone network or an IP phone network, carriers usually take countermeasures, such as adjusting or replacing the device, against the echo generation. However, it is difficult to identify which device generates the echo.
Japanese Patent No. 3310302 discloses one of the echo detecting methods for identifying an echo source, in which an echo is detected at a previous stage to every device on a communication path and an echo source is determined on the detection result.
FIG. 2A is a conceptual view of the echo detecting method as disclosed in the publication.
Echo detecting apparatuses 311a, 311b, 311c, and 311d are provided at previous stages to all of the devices 310A, 310B, 310C, and 310D on a communication path 303 similarly to that of FIG. 1. The echo detecting apparatuses 311a, 311b, 311c, and 311d monitor an upstream and a downstream signals transmitted on the communication path 303 to determine whether an echo occurs (“echo present”) or not (“echo absent”). When an echo is detected with the echo detecting apparatuses 311a and 311b and no echo is detected with the echo detection devices 311c and 311d, as shown in FIG. 2B, the device 310B is identified as a source of the echo.
However, according to the method disclosed in the publication above, echo detecting apparatuses 311a to 311d should be provided for every device on the communication path 303, leading to an increase in cost. In addition, the method has a problem that an echo source cannot be identified correctly if plural devices generate an echo at the same time.
FIGS. 3A and 3B illustrate a conceptual view and a table similar one shown in FIGS. 2A and 2B, which explain a problem in the method.
An assumption of echo occurrence in the devices 310B and 310D, such as illustrated in FIG. 3A, leads the results shown in FIG. 3B, in which a detection result of all of the four echo detecting apparatuses 311a, 311b, 311c, and 311d indicates “echo present”. In such a case, the device 310D is determined as an echo source and an echo generated in the device 310B would not be detected.
Further, as disclosed in Japanese Laid-open Patent Publication No. 2003-134005, for example, a technique called “echo canceller” has been proposed. Since the echo canceller absorbs an echo disagreeable to the ear, it is one of promising techniques. However, this technique is not intended to identify an echo source.